Process for preparing a particularly pure glycolic acid

ABSTRACT

The present invention relates to a process for preparing a particularly pure glycolic acid by saponification with chloroacetic acid with an excess of alkali metal hydroxide, with the resulting alkali metal chloride being filtered off, and the 20 to 70% strength by weight glycolic acid solution being subsequently subjected to an electrodialysis at 20° to 40° C. and a cell voltage of 0.5 to 2.5 V per cell pair. The process of the invention permits optimum removal of sodium chloride and chloroacetic acid as necessary for preparing a particularly pure glycolic acid. Other organic acids, nitrogen compounds, aldehydes and salts are not produced in this process and thus do not affect the product quality.

The invention describes a process for preparing a particularly pureglycolic acid, the purity relating to the greatest possible freedom fromimpurities such as alkali metal chlorides, organic acids, aldehydes,nitrogen compounds and salts of any type. A high-quality glycolic acidof this type is used in the pharmaceutical industry and in the cosmeticsindustry.

This purity cannot be achieved by simple distillation orcrystallization. Distillation is excluded, since glycolic aciddecomposes during this. Satisfactory purification does not occur duringcrystallization, since the above impurities such as salts and organicacids crystallize out with the glycolic acid or remain adhering to it.

The preparation of glycolic acid by saponification of chloroacetic acidwith alkali metal hydroxides has been disclosed, substantial removal ofthe resulting sodium chloride being able to be achieved by filtrationand subsequent precipitation of the sodium chloride remaining insolution by addition of methyl ethyl ketone or methyl isobutyl ketone.The organic solvent is then distilled off (DE-A-28 12 682, DE-A-28 12683). The disadvantage of this process is in the use of an organicsolvent and in that organic impurities cannot be removed in this manner.

The preparation of glycolic acid by carbonylation of formaldehyde haslikewise been disclosed (WO 92/05138). However, substantial amounts ofthe components arising as impurities, in particular formaldehyde, formicacid and methoxyacetic acid, remain in the product. Processes using ionexchangers can also be employed to remove salts. However, the necessaryregeneration of the ion exchangers makes this process very laborious andin addition produces wastewaters from the regeneration.

The preparation of glycolic acid by electrochemical reduction of oxalicacid is also described (DE 194 038). Producing a particularly pureproduct is also difficult in this process, since the by-productglyoxylic acid which behaves in a very similar manner chemically, andalso unreacted oxalic acid, make it difficult to produce high-purityquality grades.

The preparation of glycolic acid by hydrolysis of the glyconitrile,which is produced by reacting formaldehyde with prussic acid, is alsodescribed (DE-A-1254615). In this case, the nitrogen compounds arisingin the hydrolysis, in particular ammonium salts, are very difficult toseparate off, which makes producing a high-purity quality gradedifficult.

The object of the present invention was thus to provide a process whichcan be implemented on the industrial scale for preparing a particularlypure glycolic acid and which does not have the disadvantages describedabove.

Surprisingly, it has now been found that a particularly pure glycolicacid is obtained by saponification of chloroacetic acid with an excessof alkali metal hydroxide, with the resulting alkali metal chloridebeing filtered off, if the 20 to 70% strength by weight glycolic acidsolution is subsequently subjected to an electrodialysis at 20° to 40°C. and a cell voltage of 0.5 to 2.5 V per cell pair.

The saponification is carried out with an alkali metal hydroxide excessof 0 to 10, preferably 2 to 8% by weight. Of the alkali metalhydroxides, NaOH or KOH have proved to be particularly suitable. Thesaponification is advantageously carried out at a temperature of 100° to160° C., in particular 110° to 150° C. and at a gauge pressure of 0 to10, in particular 0 to 5, bar. After the saponification, only traces ofchloroacetic acids remain in the saponification product, subsequently,after filtering off the resulting alkali metal chloride, anelectrodialysis achieves a very great degree of desalting of thesolution and a further decrease in concentration of the chloroaceticacids.

The filtration which already removes the majority of the sodium chlorideis divided between saponification and electrodialysis. As a result it ispossible to recycle to the saponification the salt concentrate arisingin the electrodialysis. No process-specific wastewater is thus produced.

The further purification by electrodialysis is made possible by anexcess of alkali metal hydroxide being used in the saponification, whichexcess remains in the saponification process. Since chloroacetic acidsare stronger acids than glycolic acid, these are therefore present inthe saponification product in the ionized state as alkali metalcarboxylates and can be removed by electrodialysis.

The electrodialysis is carried out at a temperature of 20° to 40° C.,for example. The glycolic acid concentration in the diluate in this caseis advantageously 20 to 70, in particular 40 to 60, % by weight. Anapplied cell voltage of 0.5 to 2.5 V has proved to be suitable, inparticular 1 to 2 V.

The process of the invention permits optimum removal of sodium chlorideand chloroacetic acid as necessary for preparing a particularly pureglycolic acid. Other organic acids, nitrogen compounds, aldehydes andsalts are not produced in this process and thus do not affect theproduct quality.

Example 1 shows a procedure according to the process of the invention bywhich the impurities chloroacetic acid and dichloroacetic acid can nolonger be detected within the product within a detection limit of 10ppm.

After the electrodialysis, a little water is removed from the solutionby distillation and the glycolic acid concentration is set to thedesired value, customarily 70% by weight.

The invention is described by the examples below in which theelectrodialysis is carried out under the following conditions:

A laboratory apparatus from Berghof (Bel 2) is used, having a membranestack, fitted with 10 cell pairs arranged parallel, comprising anion-and cation-exchange membranes (manufacturer, Asahi Glass Co. Ltd.). Theeffective membrane area is 37 cm². The electrodes are made ofplatinum-coated titanium. An electrical voltage of 1.5 V per cell pairis applied and a flow of approximately 5 cm/s is passed over themembranes. The apparatus has 3 circulating streams. The diluate circuitstream contains the glycolic acid solution to be desalted. The aqueousconcentrate stream serves to receive the salt. The electrode flushingsolution (Na₂ SO₄, 0.2% strength by weight) prevents undesired electrodereactions, such as anodic Cl₂ development. The experimental temperatureis 20° to 40° C. All percentages in the examples are by weight.

EXAMPLES Example 1

1.00 kg of 50% strength sodium hydroxide solution (7.5% excess) is addedto 1.10 kg of chloroacetic acid having a dichloroacetic acid content of300 ppm and the mixture is heated at 115° C. for 70 h. 0.52 kg of sodiumchloride are then removed by filtration and 0.10 kg of water bydistillation. 1.41 kg of 55% strength aqueous glycolic acid remain,which contains 11% of sodium chloride and less than 100 ppm ofchloroacetic acid. This solution is fed on the diluate side to theelectrodialysis, 1.41 kg of 0.25% strength sodium chloride solutionbeing charged on the concentrate side. Over the experimental period of30 h, the voltage is kept constant at 15 V. The average electricalcurrent is approximately 0.6 A. The conductivity of the diluate solutiondecreases from 18 to 5 mS/cm and increases in the concentrate from 5 to110 mS/cm. 1.21 kg of aqueous glycolic acid solution having an acidcontent of 52% and chloroacetic acid and dichloroacetic acid contents ofless than 10 ppm are obtained. From this solution, by removing 0.31 kgof water, a 70% strength glycolic acid solution is prepared. The contentof chloroacetic acid and dichloroacetic acid is less than 10 ppm, andthe sodium chloride content is 0.02%.

Example 2

880 kg of sodium hydroxide solution (4% excess) are added to 1250 kg of80% strength aqueous chloroacetic acid solution having a dichloroaceticacid content of 240 ppm and the mixture is heated in a reactor at 145°C. at a pressure of 1.8 bar for 20 h. 476 kg of sodium chloride arefiltered off and 360 kg of water are distilled off. 1282 kg of 56%strength glycolic acid solution remain, which contains 13% of sodiumchloride and 500 ppm of chloroacetic acids. 4.3 kg of this solution arefed on the diluate side to the electrodialysis, 5 kg of 0.25% strengthsodium chloride solution being charged on the concentrate side. Over theexperimental period of 54 h, the voltage is kept constant at 15 V. Theaverage electrical current is approximately 0.6 A. The conductivity ofthe diluate solution decreases from 20 to 4 mS/cm and increases in theconcentrate from 5 to 120 mS/cm. 3.1 kg of aqueous glycolic acidsolution having an acid content of 57.3%, 0.016% of NaCl and a contentof chloroacetic acids of 380 ppm are obtained. From this solution, byremoving 0.59 kg of water, a 70% strength glycolic acid solution isprepared.

Example 3

The procedure of Example 2 is followed, but only 868 kg of 50% strengthsodium hydroxide solution (2.5% excess) are used. After filtering offthe salt, a solution which contains 57% of glycolic acid and 0.32% ofchloroacetic acid and 11.3% of NaCl is obtained. 4 kg of this solutionare thus fed on the diluate side to the electrodialysis, 4 kg of 0.25%strength sodium chloride solution being charged on the concentrate side.Over the experimental period of 48 h, the voltage is kept constant at 15V. The average electrical current is approximately 0.6 A. Theconductivity of the diluate solution decreases from 31 to 3 mS/cm andincreases in the concentrate from 5 to 115 mS/cm. 2.9 kg of aqueousglycolic acid solution having an acid content of 50.7%, 0.004% of NaCland a content of chloroacetic acid of 0.13% are obtained. From thissolution, by removing 0.76 kg of water, a 70% strength glycolic acidsolution is prepared. The content of chloroacetic acids is 1300 ppm, andthe sodium chloride content is 0.004%

Example 4

The procedure of Example 2 is followed, but an 80% strength aqueouschloroacetic acid solution having a dichloroacetic acid content of 0.25%is used. After filtering off the salt, a solution which contains 56% ofglycolic acid, 560 ppm of chloroacetic acids and 13% of NaCl isobtained. 4 kg of this solution are fed on the diluate side to theelectrodialysis, 4 kg of 0.25% strength sodium chloride solution beingcharged on the concentrate side. Over the experimental period of 50 h,the voltage is kept constant at 15 V. The average electrical current isapproximately 0.6 A. The conductivity of the diluate solution decreasesfrom 21 to 3 mS/cm and increases in the concentrate from 4 to 120 mS/cm.3 kg of aqueous glycolic acid solution having an acid content of 59.5%and containing 0.012% of NaCl and 320 ppm of chloroacetic acids isobtained. From this solution, by removing 0.42 kg of water, a 70%strength glycolic acid solution is prepared. The content of chloroaceticacid is 220 ppm, and the sodium chloride content is 0.012%.

It is claimed:
 1. A process for preparing a particularly pure glycolicacid by saponification with chloroacetic acid with an excess of alkalimetal hydroxide, with the resulting alkali metal chloride being filteredoff, which comprises subsequently subjecting the 20 to 70% strength byweight glycolic acid solution to an electrodialysis at 20° to 40° C. anda cell voltage of 0.5 to 2.5 V per cell pair.
 2. The process as claimedin claim 1, wherein the glycolic acid concentration in the diluate is 40to 60% by weight.
 3. The process as claimed in claim 1, wherein theapplied cell voltage in the electrodialysis is 1 to 2 V per cell pair.